A digital camera typically utilizes a two-dimensional array of sensors, each of which converts incident radiation into an electrical signal. The array of sensors is typically disposed at the focal point of a lens, thereby defining a “focal plane array.” Infrared cameras include sensors that are sensitive to electromagnetic radiation having wavelengths longer than about 0.8 microns. Traditionally, infrared cameras have been based upon microbolometer technology. A microbolometer is an extremely small heat sensor, comprising materials such as vanadium-oxide or amorphous silicon, whose electrical resistance changes when it receives radiation of a specific wavelength. Microbolometers, however, exhibit self-heating effects, poor sensitivity, and generate electrical noise. To overcome these problems, capacitive radiation sensors based on Micro Electro Mechanical Systems (MEMS) technology have been developed.
In a typical MEMS-based radiation sensor, a paddle or plate is supported above a substrate by thermal-bimorph support arms. At least a portion of the plate and the underlying substrate are electrically conductive, thereby serving as electrodes. These electrodes collectively define a “sensing capacitor,” the capacitance of which is a function of the electrodes' separation distance.
In operation, the plate of each sensing capacitor receives infrared radiation and heats up. The heat is conducted to the support arms, which include elements that bend in response to being heated. These elements are known as “thermal bimorphs” and their response is known as the “thermal bimorph effect.” Perhaps the most familiar thermal bimorph is the bi-metallic strip within a thermostat. As the support arms bend, the plate moves up or down (depending on the design). Movement of the plate alters the spacing between the electrodes, thereby causing a change in the capacitance of the sensing capacitor. In this fashion, radiation that is incident on the plate is sensed as a change in capacitance. The change in capacitance is captured by read-out electronics and can be quantified and interpreted to provide an image, such as in an IR camera. (See, e.g., U.S. Pat. No. 6,118,124, etc.).
Early versions of these MEMS sensors, however, were highly sensitive to changes in ambient temperature. The sensor substrate, which provides a thermal connection to the environment outside the focal plane array, provides a conduit for heat to and from the support arm. Ambient temperature changes, therefore, resulted in a perceived change in background light level or sensor sensitivity across the sensor array.
To mitigate the problems due to temperature sensitivity, thermally-compensated MEMS sensors were developed. Such sensors have been disclosed by: 1) J. Zhao in “High Sensitivity Photomechanical MW-LWIR Imaging using an Uncooled MEMS Microcantilever Array and Optical Readout,” published Mar. 28, 2005; 2) T. Ishizuya, et al., in “160×120 Pixels Optically Readable Bimaterial Infrared Detector,” published Jan. 20, 2002; and 3) Corbeil et al., in “Self Leveling uncooled microcantilever thermal detector,” published Aug. 12, 2002. In these devices, each support arm includes two thermal actuators designed to oppose one another in response to a slowly occurring change in temperature. These thermal actuators are separated by a thermal isolator, which enables the sensor to be responsive to incident radiation yet be relatively unresponsive to an ambient temperature change. Temperature compensation of up to 90-95% has been demonstrated with these devices.
Notwithstanding their improved resistance to temperature variation, prior-art thermally-compensated MEMS sensors do have some drawbacks. First, the responsivity of the opposing thermal actuators, as disclosed, is not identical. As a result, an ambient temperature change will still induce some small residual movement of the sensor paddle—as evidenced by the fact that only 90-95% compensation has been demonstrated. Second, the additional support arm structure needed to add a second thermal actuator and thermal isolator increases the area required for purposes other than radiation absorption. As a result, the optical fill-factor of prior-art thermally-compensated radiation sensors is impaired.